Superconducting nuclear magnetic resonance (NMR) magnets are usually produced from NbTi or Nb3Sn wires. Because of limitations imposed by the critical magnetic field of Nb3Sn, there is at present an upper limit of approximately 23.5 Tesla (T) for the maximum achievable field strength. This corresponds in NMR to a proton resonance frequency of 1000 MHz.
To achieve higher field strengths and/or to be able to make a magnet of a given field strength more compact, it is necessary to revert to alternative conductor materials. Research in this connection is currently mainly focused on the use of HTS tape conductors (for example ReBCO, BSCCO or iron pnictides). The magnet, however, is not produced completely from HTS materials. Rather, for reasons of cost, it is advantageous to use HTS material only for the innermost sections, and to produce the background magnet by conventional “low-temperature superconductor” (LTS) technology (that is to say with NbTi and/or Nb3Sn).
Usually, a magnet section is wound from HTS tape material, and then connected in series with an LTS background magnet. The current carrying capacity of HTS tape conductors (or their “critical current” IC) depends not only on the strength of the magnetic field in which the conductor is located but also on the orientation of the field. If the field lines run parallel to the surface of the tape, the critical current IC is high; if the field lines run at an angle to the surface of the tape, IC drops with increasing angle.
In an NMR magnet, which is generally constructed from coil sections in the form of solenoids, the field lines at the center of the coil sections run parallel to the axis of the magnet. Toward the periphery of the coil sections in the form of solenoids, the radial component of the field increases (see FIG. 3A). Here, the IC capacity utilization of the HTS conductor is accordingly at the greatest.
The current leads to the HTS section must likewise be routed through a region in which high radial field components prevail. The routing of the conductor typically takes place along a complexly shaped path, to ensure that the angle between the HTS tape and the field line is minimal.
DE 102 60 728 A1 presents a method for the ideal routing of an HTS tape conductor, as it is led out from a coil winding in a predetermined path. The underlying problem here is that the HTS tape is exposed to a curved field.
DE 10 2013 220 142 A1 discloses a conventional magnet arrangement with an HTS tape conductor and an LTS wire, which are connected electrically in series by a joint. The HTS coil forms the inner section of the coaxially arranged coils. The joint is located outside the parallel B0 field, where the magnetic field has a significantly radial component. Because of the geometry of the HTS tape conductor, the routing of the HTS tape is adapted to the local angle of the magnetic field, so that the tape plane and the field lines are aligned parallel to one another and the current carrying capacity of the HTS tape is optimized. However, here it is not discussed, or even shown, to what extent the B0 field at the peripheral regions of the solenoid winding can be influenced in order to minimize a radial component on the HTS tape conductor.
JP 2001-264402 A discloses a superconducting magnet arrangement that is likewise constructed from a number of coaxially arranged solenoid magnets. The inner coil is produced from an HTS material. The magnet is constructed in this manner to provide a magnetic field that is particularly homogeneous. For this, so-called correction coils of superconducting material are arranged outside the main coil sections, for the purpose of homogenizing the field. However, this document does not address the problem in the peripheral region of the HTS tape conductor. This is also evident from the fact that the correction coils are not arranged in the peripheral region. Furthermore, here the correction coils lie so far to the outside that they are not particularly efficient.
The solutions that are being used at present in the prior art have several disadvantages:                The IC requirement imposed on the conductor is determined by the parts of the HTS section that are exposed to great radial field components (usually at the periphery of the HTS section formed as a solenoid—that is, where the conductor is used to the greatest extent of its capacity). The resultant high requirements have an adverse effect on the price of the conductor and its commercial availability.        The routing of the conductor from and to the wound package takes place along a complexly shaped path (see for instance DE 102 60 728 A1), which is laborious and space-intensive.        In the regions of the HTS section that are exposed to great radial field components, so-called “screening currents” form in the HTS tape conductors, and for their part influence the magnetic field and cause inhomogeneities (that are highly undesired in particular in the case of NMR).        